Screening of electronic components from electromagnetic radiation, particularly from microwave and radio frequency radiation, is particularly important for components in communications equipment, radar, microwave target acquisition devices, and the like, to prevent the electronic components from external jamming or from unwanted listening devices. One approach would be to use metallic films, or coatings. However, this approach poses the additional problem of needing to physically remove the metallic screening when it is desired to send or receive a signal. Such a removal mechanism would necessarily involve a mechanical attachment, like a shutter, thereby complicating the overall design, possibly compromising the reliability of the equipment.
It would therefore be desirable to screen electronic components from unwanted electromagnetic radiation, but yet be able to remove that screen, when desired, in a simple manner.
The recently discovered superconducting materials which are superconducting above about 93.degree.K comprise multiple phases of metal oxides, the most important of which is YBa.sub.2 Cu.sub.3 O.sub.7-x (see M.K. Wu, et al. Phy. Rev. Let., 58 (9), 908-910 (1987)). Typical electrical conductivity of the YBaCuO materials above their critical temperature (T.sub.c) can be adjusted by doping. Typical values are 3.3.times.10.sup.5 siemens/M (resistivities are around 300.times.10.sup.-6 ohm-cm). The present invention is based in part on our recognition that thin films or layers of the superconducting metal oxide materials, at above their critical temperature, can transmit electromagnetic radiation, particularly microwave radiation, like sunlight, through clear glass, while below their critical temperature they serve as a reflector of such radiation. This discovery is particularly advantageous for use, for example, as an electromagnetic screen on a satellite in outer space, where the temperature of below about 93.degree.K can be attained without the use of specialized refrigeration. Thus, for example, electronic components in a satellite before launch while on the ground and/or en route to its designated orbit, may be screened from electromagnetic interference by cooling the window with liquid nitrogen. Once in orbit, temperatures below T.sub.c may be achieved naturally by shading the screen or pointing it toward deep space, thereby making the film a reflecting or absorbing screen to protect the satellite's sensors from unwarranted radiation. When warmed, however, above the transition temperature, the film would be transparent to incident radiation and would allow its transmission across the film. Heating may be achieved by pointing the screen at the sun, reflecting sunlight onto it, or by other active heating means.
The present invention is further based in part on our recognition that to serve as a temperature sensitive window-screen the thickness of a superconductor film be very small, i.e. on the order of less than about 0.001 times a skin depth, where skin depth is defined by EQU .delta.=1/.sqroot..pi.f.mu..sigma.
wherein f is frequency in cycles/sec, .mu. is magnetic permeability in henry/m and .sigma. is electrical conductivity in siemens/M.
This theory assumes that the normal temperature conductivity of the metal oxide film is below about 3.3.times.10.sup.5 (S/M). Thus, for example, at a frequency of impinging radiation of 10 GHz, the skin depth of a YBaCuO superconductor in its normal state, T&gt;93.degree.K, is about 9 micrometers. Without considering the reflections occurring at the front and rear surfaces of the skin, the thickness of the skin should be about 1 to 2 micrometer to allow sufficient transmission of the 10 GHz signal through the skin at a temperature greater than 93.degree.K. Including the front and rear surface reflections on the skin, to achieve significant transmission through the skin, the skin would have to be significantly thinner, i.e. on the order of about .01 micrometer or less.
It is therefore an object of the present invention to provide an electromagnetic shielding device which is temperature controlled to serve as a window or screen. Electrical components are shielded with a thin film or layer of superconductor metal oxide material of a sufficient thickness to allow electromagnetic transmission therethrough at a predetermined frequency, when the film is heated above the transition temperature of the superconductor.
This and other objects will be provided by the following description and from practice of the invention.